RT8070
9
DS8070-08 February 2015 www.richtek.com
©
Copyright 2015 Richtek Technology Corporation. All rights reserved. is a registered trademark of Richtek Technology Corporation.
Figure 2. Switching Frequency vs. R
RT
Resistor
Inductor Selection
For a given input and output voltage, the inductor value
and operating frequency determine the ripple current. The
ripple current, DI
L
, increases with higher V
IN
and decreases
with higher inductance
OUT OUT
L
IN
VV
I = 1
fL V
Having a lower ripple current reduces not only the ESR
losses in the output capacitors but also the output voltage
ripple. Highest efficiency operation is achieved by reducing
ripple current at low frequency, but attaining this goal
requires a large inductor.
For the ripple current selection, the value of ΔI
L
= 0.4(I
MAX
)
is a reasonable starting point. The largest ripple current
occurs at the highest V
IN
. To guarantee that the ripple
current stays below a specified maximum value, the
inductor value needs to be chosen according to the following
equation :
OUT OUT
L(MAX) IN(MAX)
VV
L = 1
fI V
Using Ceramic Input and Output Capacitors
Higher values, lower cost ceramic capacitors are now
becoming available in smaller case sizes. Their high ripple
current, high voltage rating and low ESR make them ideal
for switching regulator applications. However, care must
be taken when these capacitors are used at the input and
output. When a ceramic capacitor is used at the input
and the power is supplied by a wall adapter through long
wires, a load step at the output can induce ringing at the
input. At best, this ringing can couple to the output and
be mistaken as loop instability. At worst, a sudden inrush
of current through the long wires can potentially cause a
voltage spike at V
IN
large enough to damage the part.
Slope Compensation and Peak Inductor Current
Slope compensation provides stability in constant
frequency architectures by preventing sub- harmonic
oscillations at duty cycles greater than 50%. It is
accomplished internally by adding a compensating ramp
to the inductor current signal. Normally, the peak inductor
current is reduced when slope compensation is added.
For the IC, however, separated inductor current signal is
used to monitor over current condition, so the maximum
output current stays relatively constant regardless of the
duty cycle.
Hiccup Mode Under Voltage Protection
A Hiccup Mode Under Voltage Protection (UVP) function
is provided for the IC. When the FB voltage drops below
half of the feedback reference voltage, V
FB
, the UVP
function is triggered to auto soft-start the power stage
until this event is cleared. The Hiccup Mode UVP reduces
the input current in short circuit conditions, but will not be
triggered during soft-start process.
Under Voltage Lockout Threshold
The RT8070 includes an input under voltage lockout
protection (UVLO) function. If the input voltage exceeds
the UVLO rising threshold voltage, the converter will reset
and prepare the PWM for operation. However, if the input
voltage falls below the UVLO falling threshold voltage during
normal operation, the device will stop switching. The UVLO
rising and falling threshold voltage has a hysteresis to
prevent noise caused reset.
Thermal Considerations
For continuous operation, do not exceed absolute
maximum junction temperature. The maximum power
dissipation depends on the thermal resistance of the IC
package, PCB layout, rate of surrounding airflow, and
difference between junction and ambient temperature. The
Ω
0.0
0.4
0.8
1.2
1.6
2.0
2.4
0 300 600 900 1200 1500 1800 2100
R
RT
(k )
Switching Frequency (MHz) 1
